The present invention relates to waveform digitizing apparatus in general, and to waveform acquisition circuits in particular.
In general, oscilloscopes receive time-varying, or analog signals and graphically display the instantaneous amplitudes thereof against an internally-generated time-base sweep. These graphic displays, wherein time is plotted on the X, or horizontal, axis and amplitude is plotted on the Y, or vertical, axis, are known in the art as waveforms. Appropriate scaling of the waveform displays is provided by the oscilloscope selectable vertical gain and sweep rate controls, so that generally, a wide range of input signal amplitudes and frequencies may be accomodated. The waveform, then, is a visual representation of some electrical event, which may be either repetitive or single shot.
Waveform digitizing oscilloscopes convert the input analog signal to digital form so that a waveform may be processed by digital circuitry, which may include storage for later recall, transmission to some other location, or mathematical manipulation. The waveform acquisition is typically achieved by use of an analog-to-digital converter which samples instantaneous values of the input signal at predetermined points along the waveform and provides a digital coded signal or data words corresponding to such values. To ensure complete waveform acquisition, a predetermined number of sampling data points are provided within a time window, which window may be defined as the length of the time-base sweep, and thus the width of the time window varies in accordance with the sweep rate.
Two separate waveform acquisition techniques are generally used in present day oscilloscopes having a wide range of selectable sweep rates. These are known as real-time acquisition, wherein all of the waveform points are acquired in a single sweep, and equivalent-time acquisition, wherein all of the points are acquired one point per sweep on successive sweeps. Generally, realtime acquisition is used for the slower sweep rates where all of the points may be acquired within the conversion speed capability of the analog-to-digital converter, while equivalent-time acquisition is used for the higher sweep rates where the one point per sweep acquisition is also within conversion speed capability of the analog-to-digital converter. Because of the different mechanics involved in real-time and equivalent-time acquisition, two independent operating modes were heretofore required. The system was designed so that at lower sweep rates up to the fastest sweep rate for which real-time acquisition is feasible, i.e., within the conversion-speed capability of the analog-to-digital converter, a first operating mode would be utilized; at the higher sweep rates, the system would be switched to equivalent-time acquisition, utilizing a second operating mode, and the analog-to-digital converter would operate only once per sweep. It can be discerned that in the prior art systems there is not a smooth transition from real-time acquisition to equivalent-time acquisition as the sweep rate is increased through its range, and at the slowest equivalent-time acquisition rate wherein only one data point on the waveform is sampled each sweep, it may take a considerable amount of time to acquire all of the data points.